Create WebPage Screenshots with Node.js and SlimerJS

Last week I featured PhantomJS, a headless WebKit tool, which allows for taking screenshots, automating events on the page, and so on.  PhantomJS is an excellent tool that does so much but being locked into the WebKit engine doesn't help if you want to test other rendering engines like Firefox.  Mo engines, mo problems.

Luckily SlimerJS exists.  SlimerJS is very much like PhantomJS:  a promise-based automation system that uses Firefox's Gecko rendering engine instead of WebKit.  Let's have a look at how to automate screenshot creation using SlimerJS!

Creating a Screenshot

SlimerJS is Node.js-based so you'll write your screenshot snapping code with JavaScript:

var webpage = require('webpage').create();
webpage
  .open('https://davidwalsh.name')
  .then(function(){
    webpage.render('dwb.png', { onlyViewport: true });
    slimer.exit()
  });

With your script written you'll execute:

slimerjs take-screenshot.js

SlimerJS has its own command line tool which you'll get during install.

Setting Viewport Size

SlimerJS, much like PhatomJS, allows you to set the viewport so you can take screenshots at any size:

var webpage = require('webpage').create();
webpage
    .open('https://davidwalsh.name')
    .then(function(){
      webpage.viewportSize = { width: 1042, height: 768 };
      webpage.render('dwb.png', { onlyViewport: true });
      slimer.exit()
    });

... which is important for mobile Firefox testing as well.  I recommend setting up a script to take screenshots at all popular sizes when you hand designs over to a client or simply want to check your site's integrity!